Although the public is surprisingly ignorant of the fact, it has been common and standard practice for many decades to use electric signals in railroad tracks for controlling train operation and/or indicating position. In a simple system, a relay is held operated in series with current through both tracks of a track section and the relay is caused to be released when a train enters the track section because the train itself serves to shunt, or interconnect, the two rails of the section. The system is designed so that presence of the train releases a relay rather than causing a relay to operate. This provides a safety feature because a malfunction, such as a power failure, would more likely release a relay than operate one and it is generally considered safer to provide a false signal indicating the presence of a train than to have a false signal indicate the absence of a train.
Since rails are supported on the ground, there tends to be a leakage current between the rails which may be the result of snow, ice and/or water conditions together with other conditions which may affect the ties and/or stone ballast supporting the rails. Excessive leakage current could cause a false train present signal.
Those familiar with relays are well aware that a relay will remain operated on a current which is noticeably less than the current required to operate the relay from its unoperated position. Accordingly, it was recognized many decades ago, that under certain adverse conditions, a detecting relay might fail to release even though a train is present shunting the tracks. To reduce the possibility of having false signals of this nature, systems were introduced which provided a detecting relay which was operated and released periodically even when a train was not present. Then, when a train was present even if the shunt across the tracks was not sufficiently good to cause the release of the relay, the relay would be released by the pulsing circuit and be unable to reoperate because of the shunt on the track. Such relays have been referred to as code-responsive track relays. Although relays of this nature have been used for many decades, they do present certain problems, for example, in order to assure reliable operation, irrespective of adverse weather conditions, vibration and contaminants in the air and/or other conditions familiar to those who work with such equipment, it has been necessary to provide relays which are relatively bulky, costly, and require heavy protective housings.
Over the years, sophisticated techniques have been developed for using relays and track signals for considerably more complex functions than those thus far described. For example, the relays may be used in systems to control traffic lights and crossing arms, and/or to provide signals to or from a train cab relative to conditions ahead and/or concerning appropriate speeds. Many other applications are well-known to those familiar with railroad signaling and the applications and techniques are limited only by the fertile imagination of circuit designers.
Relays having the described characteristics are most frequently used in communication systems. Examples are railroad signal systems, as already cited; telegraph systems; teletype systems; telephone switching systems and others limited only by the ingenuity of the engineer. Accordingly, although the structure is described as used in a railroad environment, it should be understood that the structure may find utility in any system in which the available characteristics may be useful.
An example of a system using relays of the type thus far described may be seen in U.S. Pat. No. 2,934,636 issued Apr. 26, 1960 to Thomas J. Judge and assigned to the same assignee as the present invention. U.S. Pat. No. 3,879,004 issued Apr. 22, 1975 to Charles Andreasen and assigned to the same assignee discloses a somewhat more sophisticated system which will even function in a transit system that does not use steel wheels riding on a steel rail. U.S. Pat. No. 4,172,576 issued Oct. 30, 1979 to Frank A. Svet, Jr. and assigned to the same assignee illustrates an even more sophisticated system and one which utilizes many modern elements and components including solid-state devices. However, as set forth in this patent, selected critical functions are performed in response to the operation and/or release of relays similar to those described in prior patents and used for many decades. U.S. Pat. No. 4,236,093 issued Nov. 25, 1980 to David Birnbaum and assigned to the same assignee illustrates a device which can respond to the proximity of a train and which does not comprise a relay of the type which had been used for many decades. The device used is known as a Wiegand wire and is disclosed in the John R. Wiegand U.S. Pat. No. 3,892,118 issued July 1, 1975. As set forth therein the device comprises a ferromagnetic bi-stable switching module.